Extended Split-Issue: Enabling Flexibility in the Hardware Implementation of NUAL VLIW DSPs

  • Authors:
  • Bharath Iyer;Sadagopan Srinivasan;Bruce Jacob

  • Affiliations:
  • University of Maryland, College Park;University of Maryland, College Park;University of Maryland, College Park

  • Venue:
  • Proceedings of the 31st annual international symposium on Computer architecture
  • Year:
  • 2004

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Abstract

VLIW architecture based DSPs have become widespread due to thecombined benefits of simple hardware and compiler-extractedinstruction-level parallelism. However, the VLIW instruction setarchitecture and its hardware implementation are tightly coupled,especially so for Non-Unit Assumed Latency (NUAL) VLIWs. Theproblem of object code compatibility across processors having differentnumbers of functional units or hardware latencies has beenthe Achilles' heel of this otherwise powerful architecture. In thispaper, we propose eXtended Split-Issue (XSI), a novel mechanismthat breaks the instruction packet syntax of an NUAL VLIW compilerwithout violating the dataflow dependences. XSI provides a designerthe freedom of disassociating the hardware implementation of theNUAL VLIW processor from the instruction set architecture. Further,we investigate fairly radical (in the context of VLIW) changes to thehardware-like removing an adder, adding a multiplier, and incorporatingsimultaneous multithreading (SMT)-to show that ourtechnique works for a variety of hardware configurations withoutcompromising on performance. The technique can be used in bothsingle-threaded and multi-threaded architectures to achieve a levelof flexibility heretofore unavailable in the VLIW arena.